There are currently at least three types of Global Positioning System (GPS) engines used on GPS receivers for generating GPS data: Selective Availability (SA)-on, SA-on/off, and Dynamic SA-on/off. These engines attempt to compensate for the SA state of satellites and atmospheric or other satellite errors by inflating the actual Horizontal Figure of Merit (HFOM). A single aircraft may include a combination of these GPS receiver engines.
SA was turned off in May of 2000 and will continue to be off for the foreseeable future. With SA off, the GPS accuracy is possible to around 10 meters. However, the three types of GPS engines report different values for HFOM because of their differing assumptions about the satellites. The SA-on engine will report numbers with accuracy of 55 to 100 meters. The SA-on/off engine will report HFOMs of 18 to 46 meters. The Dynamic SA-on/off engine will report HFOMs of 10 to 18 meters. Although the HFOM is generally acceptable when received from the SA-on/off and Dynamic SA-on/off engines, accommodating the SA-on engine requires expanding the threshold values.
Therefore, there is a need to detect the GPS engine type of a receiver to allow receiving systems to optimally operate.